SpiNet is a neural simulation tool for large spiking networks with highly heterogeneous synapses. Neurons are modelled as I&F units with dual exponential synaptic conductances. Complex network models can be easily built using the included tool NetBuilder

CTruck 2D is a 2D, oldstyle, physically realistic 2D truck game. Predecessor of the CTruck3D vehicle simulator series. I wrote it from zero in C (+OpenGL&GLUT). It has ascii-art and nice graphics version, and a special funny version
It's a very basic thing: there are free rigid bodies and springs with strong attenuation/damping. It's very simple and very short code, and collisions of bodies with ground are good and stable.
A good simulation of this kind, however requires Lagrangian method. I proposed the Lagrange_motocross as an alternative of this kind. The code is not much longer but the it's overall more complicated, and collisions are a bad issue.